Two-piece metal cans are in widespread use in packaging a variety of domestible products, particularly beer and beverage products. Typically, such metal cans are fabricated of aluminum or steel including a one-piece open-ended can and a can end for sealing the can. During the course of manufacture the outside surface of the can is decorated (printed) with a label and overvarnished to protect the printing and the surface of the can. Additionally, in case of steel cans and some aluminum cans a coating is applied to the exterior surface of the can prior to printing. Lastly, a sanitary coating is applied to the interior of the can.
For proper application of the coatings or printing inks, it is necessary to "bake" the cans in an oven after coating or printing to achieve a proper cure. Also, the cans have to be cooled after baking in a cooler attached to the oven. For curing the exterior coating or decorative printing and overvarnish, the cans are heated in pin ovens in which a pin conveyor in the form of endless conveyor chain is fitted with a series of pins for receiving and carrying can bodies through the curing oven and through the cooler. In a typical pin oven the conveyor chain is arranged in a serpentine path in which the conveyor with cans makes several vertical passes through drying or curing zones in the oven and through the cooler. The pin conveyor with cans initially moves vertically upwardly through the first pass, reverses direction to move vertically downwardly in the second pass and so forth until the cans leave the oven and the cooler. As the cans traverse each pass, a series of hot air jets issuing from nozzles along the path of travel heat cure the exterior decorative coating or printing on the can and similarly cool the cans with cold air when passing through the cooler.
For efficient manufacturing it is desirable for the cans to move as quickly as possible through the oven. Under conventional operating conditions a pin oven will cure up to approximately 1000 to 1200 cans per minute.
Since the cans are carried on pins extending into their open ends, the cans tend to "flip-flop" under the influence of centrifugal force as they reverse direction at the end of each vertical path. Additionally, each can receives a series of pulses from curing air jets as it enters each vertical run after reversing direction. At speeds above 1000-1200 cans per minute the forces generated during flip-flop and by air jet pulses tend to inflict unacceptable damage on the cans including body damage and damage caused by cans falling off their carrier pins. Damaged cans cannot be used for packaging. Accordingly, in current practice, pin ovens are limited to this speed range.
According to present pin oven design, the oven is divided by interior walls into three chambers including a supply air plenum chamber containing hot curing air, a curing chamber, and a return air plenum chamber. The interior walls include a louver wall defining the supply air plenum chamber and for issuing curing air jets into the curing chamber, and a perforated wall defining the return air plenum chamber for receiving curing air exhausting from the curing chamber. The louver wall and the perforated wall are arranged in confronting, substantially parallel planes to define the curing chamber. The louver wall is performated in a nozzle pattern extending along each vertical path and along the circular path connecting alternate up and down passes through the oven. In the vertical paths the nozzles are arranged in typical patterns of clusters of four nozzles each spaced laterally on either side of the centerline of vertical travel and with each cluster located on given centers in the direction of conveyance. These cluster nozzle groups issued air jets impinging on the base of can bodies. In addition a series of angled nozzles are spaced in confronting relationship to the can sidewall for directing hot air toward the can wall for curing the can body. The nozzles issue discrete hot air jets which impinge on and cure surface decoration according to the oven application. Applicants have determined that the nozzle design heretofore used in fact contributes to a pulsating action of jets against can body which becomes exaggerated at higher oven speeds resulting in unstable can body motion at speeds in the 1000-1200 cans per minute range. This results in increased can damage and loss of cans falling off the carrier pins.
The circular path between vertical passes in conventional design typically contains a pair of rows of nozzles in the vertical dividing panel along inner and outer radii straddling the centerline of conveyance. By reason of the influence of centrifugal force, the circular path is one of particular turbulence for the can bodies. The existing nozzle design does little to promote can stability at high speeds in this area.
In existing ovens it is common to have the louver wall affixed to the oven walls with the rear face of the louver wall reinforced with vertically and/or horizontally arranged angle irons to withstand air supply plenum chamber pressures of up to 18 psi. A pressure drop wall may also be used behind the louver wall extending the full length thereof. In other cases no pressure drop wall is used and the reinforced louver wall forms the sole dividing member between the air supply plenum and the curing chamber. With both these arrangements the angle irons and the pressure drop plate where used contribute to turbulence in the curing air stream within theove and in the air jets issuing through the louver wall. Turbulent air jets contribute to can instability including flip-flopping on carrier pins especially at higher speeds.
In the design and construction of pin ovens, the pattern of nozzles for issuing hot air jets for curing can bodies is determined specifically for a given size can body in order to achieve optimum curing conditions for the specified can body. Additionally, the oven speed is also selected according to desired residence time of can bodies within the curing zone. Residence time is determined to a large extent by types of coatings or inks used and the type of solvents used. In practice, can manufacturers will also attempt to cure different size cans in an oven with the result that on occasion there is a serious mismatch between can size and oven speed and nozzle pattern. The mismatched cans lose stability as well as temperature uniformity as they move through the oven. Without temperature uniformity can bodies may be overcured in certain areas and undercured in other areas. As noted, can instability results in can damage.
Therefore, pin ovens currently in use are limited to a speed range of approximately 1000-1200 cans per minute. The ovens are designed for a given size of can, and as occurs in practice, ovens are used for different sized cans sometimes resulting in a mismatch between can and oven. Such mismatches result in improperly cured cans, burnt cans, and cans with damage to body or decoration due to an unstable ride through the oven.
The presently used nozzle patterns issue air jets in patterns tending to pulsate cans on their carrier pins and contribute to can instability and loss, especially at the upper range of operating speed.